Alumina trihydrate (ATH) can be used as a filler to produce coatings for paper and paperboard. Because of the relative high cost of titanium dioxide (TiO2), paper mills often replace or extend titanium dioxide with less expensive pigment alternatives, such as ATH, calcium carbonate, kaolin clays and the like. The extender can reduce or eliminate the need for the more expensive white titanium dioxide pigment. Alumina trihydrate (Al(OH)3) represents a special case among typical pigments and fillers. Alumina trihydrate (Al(OH)3) is chemically active and can react with polymers. With a Mohs' hardness of 3, alumina trihydrate is comparatively soft; the Mohs' hardness of titanium dioxide in the rutile modification, for example, is 6.5. The refractive index of alumina trihydrate is comparatively low, at n=1.57.
Paper manufacturers must be able to pump slurry from storage into the paper furnish or into the coating make-up area. In order for an ATH slurry to be considered useable as an extender pigment filler or for grinding into a TiO2 slurry, the total pigment solids content should be greater than 50 wt. %. The prior art discloses dispersed slurries that are stabilized with organic dispersants or surfactants; most of these dispersants or surfactants are anionic in nature in order to be compatible with anionic coating resins used in the paper industry. However, there are prior art cationic pigment slurries which use inorganic polyaluminum chloride (PAC) as the dispersant, but these often develop unacceptably high viscosities over time. In addition, high temperature processing conditions and subsequent thermal aging of the inorganic dispersed polyaluminum chloride slurry result in a loss of effectiveness in retaining other fillers used in the paper-makers' wet-end.
U.S. Pat. No. 2,187,050 describes aluminum salts (for example aluminum chloride) that are precipitated from solution and added to TiO2 to effect a surface coating. The neutralization process renders the alumina precipitate a neutral pH and the soluble negative counter ion (sulfate or chloride) is washed away before adding the alumina precipitate to a TiO2 slurry.
U.S. Pat. No. 4,376,655 discloses aqueous titanium dioxide slurries comprising ATH and kaolin clays. The ratio of TiO2 to alumina is between 1000:1 and 2000:1. The ATH usefully can either be a 9-10% aqueous slurry or a 50-55% dried gel. Preferably, the dried gel contains occluded carbonates. The procedure fails to produce cationic dispersed pigments.
U.S. Pat. No. 5,171,631 discloses a titanium dioxide pigment ATH extender/spacer pigment composition comprising 70-98% titanium dioxide by volume and 2-30% ATH by volume wherein the ATH has a similar median particle size as the titanium dioxide. Typically, the median particle size of the titanium dioxide is 0.2 to 0.3 microns. The ATH has a median particle size within ±20% compared to the titanium dioxide particle size. An example of a coating composition comprising the pigments was prepared with titanium dioxide and ATH and contained a cellulosic thickener, associative thickener, propylene glycol, nonionic surfactant, neutralizer defoamer, coalescing agent and biocide, in water at a solids content of 3.23%. Different types of ATH are described and the ATH that was used as a TiO2 spacer had a negative (anionic) charge in order to be compatible with anionic resin-based paints.
U.S. Pat. No. 5,342,485 discloses the use of ATH with improved whiteness in papermaking to reduce costs relative to using solely TiO2. This patent discusses the use of ATH in anionic slurries at 15-30% solids.
U.S. Pat. No. 5,824,145 discloses a photodurable titanium dioxide slurry which comprises at least 78% titanium dioxide particles and at least 3% alumina particles along with an anionic polyacrylic dispersant and additives with a pH of about 6.0 to 9.0.
U.S. Pat. No. 6,387,500 discloses coating formulations for paper and paperboards comprising aqueous slurries of titanium dioxide pigment with extender pigments, which include ATH and calcined clay, and dispersants, which include acrylates. This art is limited to anionic pigment dispersions combined with anionic binders.
U.S. Pat. No. 7,452,416 discloses the use of polyaluminum chloride added by a staged addition during TiO2 slurry grinding. Upon dilution, cationic polyamidoamine-epichlorohydrin polymer (PAE) wet strength resin was added to stabilize the slurry. The anatase TiO2 slurry grind with PAC had no specifications for controlling the grind pH and temperature. The primary stabilizing additive was an expensive organic cationic polymer.
U.S. Pat. No. 7,906,185 discloses cationic boehmite alumina used to surface treat negative charged silica to improve inkjet receptivity, enhance gloss and reduce curl.
U.S. Pat. Nos. 7,172,651 and 7,377,975 disclose pigments suitable for use in coating compositions for inkjet recording media. Surfaces of an inorganic particulate (e.g., alumina trihydrate) are interacted with a water-soluble polyvalent metal salt (e.g., aluminum chlorohydrate) in an aqueous medium. The treated particle surfaces have a “significant” cationic surface charge imparted to them. These references describe pigment surface treatments using aluminum chlorohydrate (ACH) for use in inkjet receptive paper and coatings. The ACH is highly neutralized, weakly cationic alumina that is compatible with anionic latex, having a basicity (activity) of greater than about 50%.
There remains a need to take full advantage of pigments modified with cationic dispersants to decrease the cost of achieving satisfactory opacity in paper and paper-board applications. There is a need for an improved cationic pigmented slurry for enhanced first pass retention and opacity in the sheet. There is also a need for cationic ATH slurry compositions compatible with titanium dioxide slurries that are stable at suitable viscosities. The present invention addresses these and other needs.